Human factor IX concentrates are plasma fractions which are therapeutically for patients with bleeding disorders, including hemophilia B, hemophilia A with factor VIII inhibitors, and liver disease. In some patients, with hemophilia B or liver disease the use of these concentrates has been associated with severe thrombic complications. Other investigators have demonstrated the thrombogenic potential of these concentrates, and of purified factors IXa, Xa and thrombin, in animal models. The aim of this project is the specific characterization on the activated clotting factors, particularly factor IXa, in these concentrates, and the correlation of these results with the in vivo recovery and survival of these factors and with the therapeutic effectivenesss and complications of the concentrates. Preliminary studies, using a new, specific assay for factor IXa, have demonstrated the presence of factor IXa in activated concentrates, which have been specially prepared for use in factor VIII-inhibitor patients. This project will attempt to identify whether or not factor IXa is also present in the routinely prepared commercial concentrates. A specific assay for factor VIIa, using coupled amidolytic and clotting assays, and an amidolytic assay for factor Xa, will also be used to quantitate factors VIIa and Xa in these concentrates. These specific assays can also be performed on plasma. A major goal of this project will be to correlate the content of activated factors in concentrates with: 1) the presence of these factors in post-infusion plasma samples from patients transfused with these concentrates and 2) the clinical response of the transfused patients. The identification of the specific activated factor(s) in factor IX concentrates responsible for thrombotic complications versus hemostatic benefit has great practical importance, since it would provide a rationale for modifying the purification procedures to adjust the content of the pertinent factor(s). This project should also contribute to the broad aim of understanding the process of thrombosis.